Efficient and tunable interface for quantum networks

24.05.2012

While several building blocks for a quantum computer have already been successfully tested in the laboratory, a network requires one additonal component: a reliable interface between computers and information channels. In the current issue of the journal Nature, physicists at the University of Innsbruck report the construction of an efficient and tunable interface for quantum networks.

Image: At the core of the experiment lies an optical resonator consisting of two highly reflective mirrors. (Photo: C. Lackner)

Quantum technologies promise to redefine the landscape of
information processing and communication. We already live in an information age, in which vast amounts of data are
sent around the world over optical fibers, but future quantum networks may be
many times more powerful. These
networks will require interfaces that can transfer information from quantum
processors onto light particles (photons). Such interfaces will allow optical fibers to transmit
information-bearing photons between remote data registers, which are likely to
be composed of quantum dots or ions.
In contrast to classical information, quantum information can’t be copied without being corrupted. Instead, physicists around the
world are searching for ways to transfer quantum information between matter and
light using entanglement, the quantum property in which the state of one
particle depends on the state of a second. Now, a research team led by Rainer Blatt, Tracy Northup, and
Andreas Stute at the University of Innsbruck’s Institute for Experimental
Physics has demonstrated the first interface between a single ion and a single
photon that is both efficient and freely tunable.

High efficiency and precision

The Innsbruck physicists trap a single calcium ion in a so-called
Paul trap and place it between two highly reflective mirrors. They excite the ion with a laser, thereby generating a photon which is
entangled with the ion and which is reflected back and forth between the
mirrors. Custom tuning of the entanglement
between ion and photon is possible by adjusting the frequency and amplitude of
the laser. This technique has two
significant advantages over previous approaches that have entangled atoms with
light: “The efficiency with which we produce entangled photons is quite high and
in principle could be increased to over 99 percent,” explains Northup. “But above all, what this setup lets us
do is generate any possible entangled state.” To this end, the frequency and amplitude of the laser light
are carefully chosen so that target collective state of the ion and photon is
reached. At the core of the experiment
lies an optical resonator consisting of two highly reflective mirrors. Photons bounce back and forth up to 25,000
times between these mirrors, interacting with the ion, before escaping through
one mirror into an optical fiber.
“Along with an efficient entanglement process, we’ve demonstrated an
entangled quantum state between an atom and a photon with the highest precision
measured to date,” explains Andreas Stute.

Technology for the
future

The experiment offers important insights into the interaction
of light and matter and may prove useful in constructing quantum computers or a
future quantum internet. “Whenever
we have to transfer quantum information from processing sites to communication
channels, and vice versa, we’re going to need an interface between light and
matter,” explains Northup. The researchers are supported by the Austrian Science Fund (FWF) and the
European Union. Their results
appear in the May 24 issue of Nature.